This invention relates to a process for producing nylon, and is more particularly concerned with preventing deposits of polymer from steam which is vented during preparation of nylon from monomer in the form of aqueous salts of amines and carboxylic acids.
The term "nylon" is defined herein as in the American Society For Testing And Materials, Designation D 123-74a, "Standard Definitions of Terms Relating To Textile Materials" (1974), as a generic term for any long-chain synthetic polymeric amide which has recurring amide groups as an integral part of the main polymer chain, and which is capable of being formed into a filament in which the structural elements are oriented in the direction of the axis. The term "66-nylon" refers to polyhexamethyleneadipamide.
In the condensation polymerization process for producing nylon from an aqueous salt of a diamine and a dicarboxylic acid, or aqueous .omega.-aminocarboxylic acid, the reaction mixture is heated under autogeneous pressure to remove water present at the start and formed by condensation reaction. The water is discharged as steam through a pressure-control valve and vent pipes. In a typical autoclave cycle for producing 66-nylon from hexamethylene diamine and adipic acid, the aqueous salt may be heated until the steam pressure reaches at least 250 psig (17.5 kg./cm.sup.2). Steam is then released through a valve and vent pipe at a rate which maintains constant pressure in the autoclave. Later in the polymerization cycle, the temperature is raised and the pressure is lowered to continue the reaction.
The aqueous salt solution boils vigorously during this process. Molten nylon of low viscosity becomes entrained in the steam vapor. As the steam vapor and entrained aerosols pass through the valve, the pressure decreases suddenly. The saturated steam becomes dry, superheated steam and the nylon aerosols rapidly release water due to the sudden pressure drop and to exposure to the drier steam atmosphere. This results in rapid cooling of the molten aerosol with a concurrent increase in viscosity. The viscosity also increases due to further polymerization of the aerosol from a reduction in water content.
The cooled and viscous entrained aerosols stick to and solidify on the low pressure side of the valve and on the dry vent walls. The aerosols continue to polymerize and eventually degrade. The heaviest accumulation is generally close to the valve. Within a few weeks to a few months, the accumulation of polymer builds up to restrict the flow of vapor from the processing vessel, and absolute pressure control becomes difficult. This requires shutting down the processing equipment with a loss of production, and high maintenance costs for cleaning the vent line and the process equipment. In many commercial installations, dual vent lines are provided. This avoids the requirement for shutting down the process equipment when one vent line must be cleaned, but requires additional investment and still involves high maintenance costs on the vent lines.